Marine cycloidal propellers were originally developed in the early 1920's and are commonly known as Voith-Schneider propulsion systems. The cycloidal propeller derives its name from the cycloidal path that individual propeller blades on its propeller hub circumscribe as the propeller moves through the water. These unique propellers have blades which extend parallel to the rotational axis of the propeller hub and are pivotal about discrete blade axes parallel to the propeller rotational axis. In propulsion systems of this type, the direction and magnitude of thrust may be varied along any line normal to the rotational axis of the hub, thereby obviating the need for directional rudders. In contrast, traditional helical-screw propellers have blades which are perpendicular to the rotational axis and can only vary the magnitude of thrust along their axis, thereby requiring rudders to change the direction of thrust.
In a cycloidal propeller assembly, the propeller blades operate as lifting surfaces. The angles of attack of these blades continuously varies from zero to a maximum in the desired direction of travel, then back to zero during each forward and aft half-cycle or half propeller rotation. The lift force varies as each blade circumscribes the orbital path and the time average of this force vector, for all blades over a complete revolution, is equal to the direction and magnitude of the propeller thrust. The blades which normally project downward into the water flow ape usually mounted in a recessed rotating propeller hub which orbits about an axis normal to the hull.
Cycloidal propellers generally include an offset control mechanism, a power drive and gear reduction system, and a rotating propeller hub which includes the variable-pitch propeller blades. In existing cycloidal propellers, the offset control mechanism is on top of the drive system and consists of two orthogonal hydraulic servo motors that pivot a vertical control rod-end off its concentric axis with the rotating propeller, and levers the lower control rod-end in the rotating propeller hub, in a direction perpendicular to the direction of thrust and at a selected magnitude offset. This action, in turn, causes an offset ring in the lower rotating hub to continuously adjust the various blade bell-cranks and/or crank-type kinematics to cyclically vary each variable-pitch blade angle. The offset control mechanism, the mechanical kinematics blade-pitch linkage systems and the propeller hub are large, complex and mechanically inefficient.